Server-side JavaScript

TeselaDB runs the JavaScript of MongoDB queries: $where, $function, $accumulator and mapReduce. It does so with its own interpreter written in Rust that only sees the documents it receives.

db.orders.aggregate([
  {
    $addFields: {
      vat: {
        $function: {
          body: "function (total) { return Math.round(total * 0.21 * 100) / 100 }",
          args: ["$total"],
          lang: "js",
        },
      },
    },
  },
]);

Limits

Each run has step, memory and time limits, as in MongoDB. JavaScript local dates are in UTC and BSON types show with their MongoDB names (NumberLong, ObjectId…).

Turn it off

Start the nodes with --noscripting (or security.javascriptEnabled: false in the configuration file) and queries with JavaScript answer with an error.

mapReduce is deprecated in MongoDB, but it stays because mongosh and the drivers still send it. For new code use the aggregation framework.